Here is a Technical Service Bulletin from Ford that pertains to your truck for repeat heater core leaks. At the end of the tsb it talks about installing a restictor in the heater hose. This restrictor will prevent excess pressure from going through the heater core. I would check the other items out that it talks about also.
CLIMATE CONTROL - REPEAT HEATER CORE FAILURE
COOLING SYSTEM - REPEAT HEATER CORE
1985-2002 CROWN VICTORIA, ESCORT, MUSTANG
2000-2002 ESCORT ZX2, FOCUS
1985-1990 BRONCO II
1985-1997 F-250 HD, F-350
1985-2002 ECONOLINE, F-150, RANGER
1988-1997 F SUPER DUTY
1999-2002 SUPER DUTY F SERIES
2001-2002 ESCAPE, EXPLORER SPORT TRAC, EXPLORER SPORT
1985-1992 MARK VII
1985-2002 CONTINENTAL, TOWN CAR
1993-1998 MARK VIII
1985-2002 GRAND MARQUIS
Some vehicles may exhibit (repeat) heater core leaks. This may be caused by a chemical reaction called electrolysis. Electrolysis involves an ion exchange between the heater core and engine coolant which can result in a breakdown of the heater core material. This is similar to the operation of a battery.
Check for electrolysis on any vehicle with a heater core failure. If electrolysis is verified, flush the coolant and follow additional steps as required. Refer to the following Service Procedure for details.
If there is a condition of a heater core leaking or repeal heater core leak, check for electrolysis using the following procedure:
1. To check for electrolysis use a DVOM set on DC volts. Place the positive probe of the meter in the engine coolant and the negative probe on the negative battery post.
2. Adjust engine throttle to 2000 RPM to properly get coolant flow and true electrolysis voltages.
3. If more than .4V is recorded, flush the coolant and recheck (follow guidelines in TSB 98-23-16 for Cougar). See Coolant Fill Procedure below to remove trapped air on 4.6/5.4/6.8L modular engines. NOTE
EXPORT MARKETS, BE SURE THE WATER IS DESALINATED.
4. If there is still excessive voltage present in the coolant, check the engine to body/battery grounds. Also, verify proper grounding of any aftermarket electrical/electronic equipment which has been installed into the vehicle. Improperly grounded electrical devices can cause electrolysis to occur.
5. If the condition is still present after the grounds have been checked, it may be necessary to add extra grounds to the heater core
and engine. A hose clamp can be used to secure a 16 AWG stranded copper wire to the heater core inlet tube. The other end should be secured to an EXISTING FASTENER on the body sheet metal. Extra grounds to the engine should be attached between EXISTING FASTENERS on the engine and body sheet metal. Verify continuity of any added grounds to the negative battery terminal.
6. If the condition is still present, add a restrictor (part F1UZ-18D406-A) on the inlet hose with the arrow facing the direction of coolant flow (toward heater core
). Cut the line and install with 2 hose clamps. It is important that the restrictor be installed in the right direction of flow and as close to the engine block as possible (not near the heater core itself).
Coolant Fill Procedure
At times, in order to completely remove any trapped air in the cooling system of vehicles equipped with 4.6/5.4/6.8L modular engines, it may be necessary to use the following procedure:
1. Disconnect the heater hose at the right front or rear of the engine.
2. Remove the thermostat
3. Using the thermostat
opening, carefully fill the engine with the proper clean coolant mixture until observed at the engine side heater hose
4. Reconnect the heater hose
and reinstall the thermostat
5. Fill the degas bottle to the coolant fill level mark.
6. Run the engine until it reaches normal operating temperatures.
7. Select max heat and max blower speed on the climate system. NOTE
IF THE HEAT OUTPUT IS INSUFFICIENT, OR THE ENGINE DOES NOT REACH NORMAL OPERATING TEMPERATURES, VERIFY PROPER THERMOSTAT OPERATION AND REPEAT PROCEDURE IF REQUIRED.